Magneto-optic recording media are also known by several other names: thermomagnetic media, beam addressable files, and photo-magnetic memories. All of these terms apply to a storage medium or memory element which responds to radiant energy permitting the use of such energy sources as laser beams for both recording and interrogation. Such media modify the character of an incident polarized light beam so that the modification can be detected by an electronic device such as a photodiode.
This modification is usually a manifestation of either the Faraday effect or the Kerr effect on polarized light. The Faraday effect is the rotation of the polarization plane of polarized light which passed through certain magnetized media. The Kerr effect is the rotation of the plane of polarization of a light beam when it is reflected at the surface of certain magnetized media.
Magneto-optic recording media have several advantages over known magnetic recording media:
1. The spacing between the medium and recording head is greater, thus reducing potential for contact and
2. Using a pulsed laser beam as the writing means, very high density data storage is possible.
3. With a protective layer on top of a magneto-optic layer, the medium is affected less by dust than magnetic media.
In magneto-optic recording, data is written into medium having a preferentially directed magnetization by exposing a localized area (spot or bit) on the recording medium to an electromagnetic or other energy source of sufficient intensity to heat the recording medium above its compensation or Curie point temperature and simultaneously biasing the medium with a magnetic field. Preferably, the energy source is a laser which produces a monochromatic output beam. The magnetic field required to reverse the magnetization of the recording medium varies with the temperature to which the recording medium is brought. Generally speaking for a given material, the higher the temperature, the smaller the required magnetic field coercive force.
The write or record operation for both Curie point and compensation point writing is as follows:
1. The medium is initially in a randomly magnetized state. A domain will herein refer to the smallest stable magnetizable region, although in common usage, a domain is a uniformly magnetized region of any size. A selected area of the medium may be magnetized by exposing said area to a continuous energy beam and a small magnetic bias field normal to the surface of the medium.
2. A small magnetic bias field oriented perpendicular to the surface or plane of the film, but oppositely directed to the magnetic field applied earlier is applied over the entire thin film medium.
3. With the biasing field in place, a light beam from a radiant energy source such as a laser beam is directed toward a selected location or bit on the film where it causes localized heating of the film to a temperature at or above the compensation temperature. When the laser beam is removed, the bit cools in the presence of the biasing magnetic field and has its magnetization switched to that direction. The medium, in effect, has a magnetic switching field which is temperature dependent. The magnetic biasing field applied to the irradiated bit selectively switches the bit magnetization, with the bit momentarily near its compensation temperature under the influence of the laser. The momentary temperature rise reduces the bit coercive force.
In the write operation, the write laser beam (e.g., about 8-12 mW) is focused to the desired diameter (e.g., 1.0 microns) onto the surface of the recording medium by an objective lens.
The memory element or recorded bit is interrogated, or read, nondestructively by passing a low-power (e.g., 1-3 mW) beam of polarized light (e.g., a laser beam) onto the bit storage site for a sufficiently short time so as not to heat the medium to change its magnetic state. The read laser beam is normally shaped to a circular cross-section by a prism, polarized and focused to some small diameter (e.g., 1.0 micron) onto the recording medium by a lens. When the read beam has passed through the recorded spot, it is sent through an optical analyzer, and then a detector such as a photodiode, for detection of any change or lack of change in the polarization.
A change in orientation of polarization of the light is caused by the magneto-optic properties of the material in the bit or site. Thus, the Kerr effect, Faraday effect, or a combination of these two, is used to effect the change in the plane of light polarization. The plane of polarization of the transmitted or reflected light beam is rotated through the characteristic rotation angle .theta.. For upward bit magnetization, it rotates +.theta. degrees and for downward magnetization -.theta. degrees. The recorded data, usually in digital form represented by logic values of 1 or 0 depending on the direction of bit magnetization, are detected by reading the change in the intensity of light passing through or reflected from the individual bits, the intensity being responsive to the quantity of light which is rotated and the rotation angle.
Erasure can be accomplished by simply writing new information over old portions of the medium or by simply exposing any given bit to a laser beam of sufficient intensity and then cooling that bit in the presence of a magnetic field in the direction of the initially applied magnetic field. Generally, in the recording process, the external biasing magnetic field is applied by a magnet set above or behind the magneto-optic medium, and in the erasing process, the magnet is reversed in direction.
Forty-five decibels in a 30 kHz band width is generally considered the minimum CNP acceptable for direct read after write (DRAW) media. The speed at which the bits can be interrogated and the reliability with which the data can be read depends upon the magnitude of the magneto-optic properties, such as the angle of rotation, of the thin film and upon the ability of the interrogation system to detect these properties.
The main parameters that characterize a magneto-optic material are the angle of rotation, the coercive force (H.sub.c), the Curie temperature and the compensation point temperature. The medium is generally comprised of a single layer or multiple layer system where at least one of the layers is a metal alloy composition. Binary and ternary compositions are particularly suitable for amorphous metal alloy formation. Suitable examples would be rare earth-transition metal (RE-TM) compositions, such as: Gadolinium-cobalt (Gd-Co), Gadolinium-iron (Gd-Fe), Terbium-iron (Tb-Fe), Dysprosium-iron (Dy-Fe), Gd-Tb-Fe, Tb-Dy-Fe, Tb-Fe-Co, Terbium-iron-chromium (Tb-Fe-Cr), Gd-Fe-Bi (Bismuth), Gd-Fe-Sn (Tin), Gd-Fe-Co, Gd-Co-Bi, Gd-Dy-Fe and Tb-Fe-Co-Cr.
Japanese Patent Application No. 59/206864 discloses an optical magnetic recording medium consisting of an amorphous alloy and film. The film has an axis of easy magnetization in the vertical direction against the film face. The optical magnetic recording medium consists of a binary composition which in the medium is described as having a composition of Tb.sub.O.25 (Fe.sub.O.85 Co.sub.0.15).sub.0.71 Cr.sub.0.04 with a film thickness of 1000 A. The film is deposited using a sputtering process with a sputtering gas pressure of 3-8 Pa.
Japanese Patent Application No. 59/88076 describes the terbium-iron-cobalt-M alloy composition for use in a magnetizable amorphous thin film layer (where M is at least one metal selected from tin, bismuth, nickel, chromium and copper). The alloy composition is: (Tb.sub.x (Fe.sub.1-y Co.sub.v).sub.1-x).sub.1-z M.sub.z (x is from 0.1 to 0.4. v is from 0.01 to 0.5. z is from 0.002 to 0.1). The amorphous magneto-optic layer is formed to 0.01-1 micron thick on an appropriate substrate such as glass by vacuum deposition or sputtering.
Magneto-optic amorphous thin films having a terbium-iron-cobalt composition can be typically deposited by a triode sputtering process. Such sputtering process conditions are an initial vacuum of 4.4.times.10.sup.-7 mBar and a background operating pressure resulting from the sputtering gas (argon) of 1.2.times.10.sup.-3 mBar. Initial high vacuums were needed to minimize contaminants. Relatively low sputtering argon pressure was needed to increase the mean free path from the sputtering target to the deposition substrate such that contaminant interactions were minimized. Such a deposition process is disclosed in the Freese et al U.S. Pat. No. 4,569,881, which is incorporated herein by reference.
To achieve such deposition background pressures, the vacuum chamber needs to be pumped for at least four hours and is typically pumped overnight, resulting in pumping times of 16 hours or greater. Such evacuation times result in high energy costs and manufacturing inefficiencies.